The present invention relates to an electrode assembly and, more particularly, to a disposable leg plate electrode assembly, having a floating maternal reference electrode, which is most useful as part of a fetal monitoring system.
Fetal monitoring is a technique which has been used for many years, primarily when a mother is giving birth. The technique is accomplished by attaching one or more electrodes to the fetus. An additional electrode attached to the mother is used to establish a base or reference voltage for the fetal electrodes. The fetal electrodes, via leads passing through the birth canal, and the reference electrode are connected to a fetal monitoring device.
In order to provide an electrical and mechanical connection between the fetal electrodes and the fetal monitoring device, obtain a maternal reference for reliable signal processing, and prevent disengagement of the fetal electrodes in the event that the mother moves relative to the fetal monitoring device, known electrode assemblies use a metal base plate strapped to the mother's thigh. The plate carries a pair of insulated connectors which engage the leads passing through the birth canal and connecting to the fetal electrodes. A main cable electrically coupled to the metal base plate and to the connectors is used to link the electrode assembly to the fetal monitoring device.
Although the electrode assembly described above has had some success, it has several drawbacks. The most significant of these drawbacks is the risk of cross-contamination presented by the assembly, which operates in an environment filled with body fluids. A related drawback is its expense: the cost of such assemblies normally is between $50 and $200. Moreover, although they may be reused, the assemblies represent a capital investment to a hospital or clinic which cannot be directly charged to the patient. The expense is increased because the plate assemblies become soiled during use and must be cleaned before being reused. In addition, a hospital or clinic would require numerous identical assemblies to assure availability during the cleaning process.
Thus, there is a continuing need for high quality, sanitary, but inexpensive electrode assemblies. For purposes of convenience and safety (e.g., to maintain sterility in a medical environment), the electrode assembly should be sufficiently inexpensive to manufacture that it is practical to dispose of and to replace the electrode assembly after only one use (hence, the electrode assembly must be "disposable").
Conventional electrode assemblies, such as that described above, are relatively complex in their structure. Many of them have hard, bulky components which make them uncomfortable to the patient. They often have a relatively high profile (height). The connectors project from the electrode assembly a significant distance to allow connection of the lead wires from the fetus. A high profile is disadvantageous because it increases the risk of damage by hitting other objects during use and storage. The electrical contacts between the fetus and the connectors and between the reference electrode and the patient are essential; those contacts must be protected from disengagement. Consequently, a low profile electrode is preferable.
Most conventional electrode assemblies suffer from motion artifacts. Motion artifacts can be defined as motion-induced fluctuation of skin potential. Such artifacts create electrical interference which is often superimposed on the bipotential skin signal measured by the electrode, thereby reducing the electrode's usefulness as a diagnostic tool. Motion artifacts have long been a problem in measuring biopotentials. Artifacts are generally caused by movement of the patient relative to the electrode applied to the patient's skin. That movement disturbs the skin potential and creates extraneous output on the monitor which either masks the desired bipotential signal or shifts the base line.
The conventional electrode assemblies are typically attached to the patient on the mother's thigh. That location of attachment and the metal base plate component of the assemblies give such assemblies the name "leg plate." (Although the present invention neither incorporates a metal base plate nor must be attached to the patient's leg, as discussed below, the name "leg plate" is retained for purposes of continuity.) Encircling the thigh and assuring attachment of the conventional assembly are relatively untidy and unsanitary straps. It would be desirable both to avoid these straps and to provide the electrode assembly with sufficient flexibility to permit attachment to the patient at a variety of locations, including the thigh, the abdomen, and other sites.
As the above discussion makes evident, the problem of providing a highly reliable, disposable, low-profile, comfortable electrode assembly which minimizes motion artifacts has presented a major challenge to designers in the labor and delivery health care field. The development of an economical electrode assembly having these qualities would represent a major technological advance in the field. The advantages of such a device would satisfy a long-felt need within the medical profession.
U.S. Pat. No. 4,209,020 issued to Nielsen discloses an electrode assembly which attempts to satisfy that need and to overcome the drawbacks of conventional devices. The Nielsen assembly is disposable and less expensive than those devices described above. Nevertheless, it still has practical problems, the three most significant of which are: 1) the way in which the leads from the remote fetal electrodes are attached to the electrode assembly, 2) the way in which the electrode assembly is attached to the patient, and 3) the way in which the maternal reference electrode is attached to the electrode assembly.
Nielsen uses spring clips to attach the fetal electrode leads to the assembly. These protruding clips give the assembly a high profile and complicate both the use of the assembly and its manufacture. As a general rule, of course, the more complicated the manufacturing procedure, the more expensive the item. Nielsen also uses an adhesive to attach the assembly to the patient. An adhesive attachment on the patient's skin precludes easy adjustment of the assembly, is uncomfortable for the patient, and risks injury to the patient upon removal. Finally, Nielsen fixes the reference electrode to the body of the electrode assembly. Because the maternal reference electrode is mechanically coupled to the assembly, the risk of motion artifact as the mother moves relative to the reference electrode is great.
A vastly improved electrode assembly is disclosed in U.S. Pat. No. 5,197,472 issued to DiSabito. The disposable electrode assembly includes a pair of thin, flexible, foam pads having adhesive on their facing surfaces. Leads from the fetal electrodes are secured between the pads by the adhesive and are in electrical contact with conductive plates also located between the two pads. The lower pad carries a reference electrode, fixed to a conductive plate, which is in electrical contact with the patient when the assembly is attached to the patient. Such attachment is assured without using an adhesive on the lower pad which contacts the patient's skin. The conductive plates and the reference electrode are electrically connected to a fetal monitoring device by thin conductors also secured between the pads by the adhesive. The thin conductors combine in a cable assembly adapted to connect to the fetal monitoring device.
It would be desirable to mechanically decouple the maternal reference electrode from the electrode assembly to reduce motion artifacts. It would also be desirable to provide an electrical and mechanical connection, between the fetal electrode wires and the electrode assembly, which allows quick and easy attachment of the wires, allows multiple attachment to the electrode assembly, and allows one or more of the wires to be removed easily and without disturbing the connection of the remaining wires.
Therefore, to overcome the shortcomings of the existing medical electrode assemblies and to satisfy the need of the medical profession, a new, reliable, disposable, low-profile, comfortable electrode assembly is provided. A primary object of the present invention is to provide an improved electrode assembly which can be manufactured at low cost yet meets the patient's needs, especially comfort and the assurance of a continually functional device. A related object is to provide an electrode assembly which is easy to use.
It is still another object of the present invention to reduce motion artifacts. An additional object is to assure that the electrode assembly has a very low profile. Yet another object of this invention is to allow for quick and easy attachment and detachment, both between the electrode assembly and the patient and between the fetal electrodes and the electrode assembly. Both types of attachment and detachment should be sufficiently simple that they can be done in low lighting.